Comparing Apples with Apples
You cannot leave Graphic Designer without comparing it with other competing products. The product that comes closest is Macromedia Fireworks. Fireworks has currently just released its seventh version, called Fireworks 8 (it is a marketing thing to get numbers wrong). Many of the features you see in Fireworks are in Graphic Designer. There are areas—such as support for CSS, scripting, and third-party extensibility—that Fireworks has long supported that are either missing or shortchanged in Graphic Designer. With that said, Graphic Designer is, in my opinion, superior to Fireworks as an illustration tool. Fireworks is designed specifically for the Web, whereas Graphic Designer allows you to create graphics for print as well as the Web.
When you are working with graphics it is also very hard not to cover Adobe Illustrator or PhotoShop. Both products are so deeply entrenched with the creative community that a new product will have a hard time even if were on the same level as Adobe's products. I think it may take a couple more iterations for Expression Graphic Designer to get there.
All in all, Microsoft has produced a very good first version program. The faithful will be won over as new releases come out.